Because of its excellent chemical and physical properties, silicon carbide has potential applications as structural materials for use in high temperature environments, sliding materials or corrosion-resistant materials, which include mechanical seal rings, plungers, bearings, sand blast nozzles, tappets and microwave absorbers.
Silicon carbide sintered product is conventionally produced by reaction sintering, hot pressing or pressureless sintering. Of these methods, the pressureless sintering is most commonly used. In order to improve the sinterability of silicon carbide, various sintering aids have recently been developed.
Two typical methods which have been developed for the sintering of silicon carbide are hereunder described according to crystal form.
First, Japanese Patent Application (OPI) No. 148712/76 corresponding to U.S. Pat. No. 4,124,667 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") describes a method wherein .alpha.-silicon carbide is mixed with 0.15 to 3.0 wt% of boron, 0.5 to 5.0 wt% of a carbonized organic material and up to 1.0 wt% of an additional carbon. The mixture is molded into a suitable shape and then the molded product is sintered so as to provide a density of at least 2.4 g/cm.sup.3 which is greater than 75% of theoretical density.
Second, Japanese Patent Application (OPI) No. 78609/75 corresponding to U.S. Pat. No. 4,004,934 describes a process for producing sintered silicon carbide with a density not smaller than 85% of theoretical density by sintering a molded mixture of .beta.-silicon carbide with a boron compound corresponding to 0.3 to 3.0 wt% of boron and a carbonaceous additive corresponding to 0.1 to 1.0 wt% of carbon. According to the specification, the function of carbonaceous additive is to reduce silica which always is present in silicon carbide powder in small amounts or which forms on heating from oxygen absorbed on the powder surfaces. The other function of the carbonaceous additive is that it acts as a getter for free silicon. If this additive is incorporated in an amount of more than 1.0 wt%, the unreacted excess carbon tends to form voluminous grains in the sintered silicon carbide that act much like permanent pores and such excess thereby limits the ultimate achievable density and strength.
However, whether the silicon carbide is in the .alpha.- or .beta.-form, if sintering is effected at temperatures higher than 2,000.degree. C. in an attempt to obtain a dense product, .beta.-silicon carbide experiences grain growth as it is transformed to .alpha.-silicon carbide. Further, .alpha.-silicon carbide also undergoes grain growth although no phase transformation occurs. In any event, it has been impossible to obtain a dense and microfine sintered product by any of the two prior art techniques.